Process for the oxidation of ammonia and improved catalyst therefor



Patented June 11, 1935 UNITED STATES PROCESS FOR THE MONIA THEREFOR OXIDATION OF AM- IMPROVED CATALYST John N. Tilley, Woodbury, and Harold White head, South Orange,

half to ,E. I. du Pont de a corporation of Delaware,

Wilmington, Del., and one-half Newark, N. J.,

to Baker and Company, a corporation of New Jersey No Drawing. Application July 22, 1933,

N. J., assignors of one- Nemours 85 Company,

Inc.,

Serial No. 681,756

\ 13 Claims.

This invention relates to a process for oxidizing ammonia to form oxides of nitrogen, and more particularly to improved catalysts for such a process. i

In the most satisfactory processes for this reaction carried out heretofore, the gas mixture consisting of ammonia and an oxygen-containing gas, as for example air, has been passed, at an elevated temperature, through a catalyst, preferably composed of platinum, or a platinum alloy, in the form of a fine wire gauze. While the temperature employed in the oxidation'of ammonia may vary rather widely, it should fall within the limits of 650 C. (1202 F.) to 1200 C. (2192 F.) Various alloys of platinum with other metals, particularly with other metals of the platinum group, have been proposed for use in the oxidation of ammonia. A marked advance was made by Davis (U. S. Patent 1,706,055) who made use of a catalyst comprising an alloy of platinum with rhodium. This alloy has an advantage over pure platinum in that it brings about higher conversion efficiency, is of longer life, and results in much lower volatilization losses of the catalyst. As a general rule diflicultics are frequently encountered with these alloy catalysts, under certain conditions, in initiating the oxidation reaction, particularly when operating the catalyst for the first time in the ammonia oxidation process.

The object of this invention is an improved process for the oxidation of ammonia. A further object is such a process utilizing a superior form of catalyst. A still further object is the oxidation of ammonia with the use of a more efficient catalyst, whereby the ease of initiation of the reaction is increased, as well as the rate of initial activation of the catalyst. Additional and further objects will be apparent as'the description of our invention proceeds.

We have foundthat the foregoing objects are accompl shed by the use, as a catalyst in the oxidation o ammonia, of an alloy contaning more than one metal of the platinum group, said alloy being coated with pure platinum. Preferably, such alloy will be an alloy of platinum with rhodium, since these alloys have been foundto be particularly effective in ammonia oxidation. Alloys of platinum with palladium ruthenium, and other platinum metals, however, may likewise be coated with pure platinum for such use.

Our invention is not limited to any particular method of applying the platinum coating to the alloy catalyst, but preferably such coating will be accomplished by the process of electroplating, since by such a method amore satisfactory and uniform deposition of the platinum is obtained, 1. e., a solid layer or coating of pure platinum is obtained as distinguished for example from a porous coating of platinum sponge or platinum black. For most satisfactory results, the platinum will be present ont the surface of the platinumrhodium alloy in an amount equal to 0.1 to 1.0% by weight of the entire catalyst mass. Various compositions of the platin -rhodium alloy may be employed as the material on which the platinum coating is deposited, but we prefer to use alloys having a rhodium content between 0.5% and 50.0%. We find a 90-40 platinum-rhodium alloy a very satisfactory basefor the coating of pure platinum.

While we have described our invention with particular reference to the use'of wire gauze catalysts, we may, if desired, employ the alloy in the form of perforated sheets of metal, narrow strips, or small particles of various descriptions as catalyst, when coated with pure platinum.

We have found that when an alloy of platinum and rhodium, electroplated with pure platinum, is used, the catalyst is lighted and the reaction initiated more readily than when the catalyst comprises an uncoated alloy of platinum and rhodium. Likewise, the time required to reach maximum conversion with a new catalyst is greatly diminished by the presence of the platinum coating, with a resulting saving in time and yield, while the conversion efliciency is of the same high order as obtained with the uncoated platinumrhodium alloy.

In many cases, it has been found difflcult to bring a new platinum-rhodium catalyst to its proper operating efficiency withinva short time. Platinum catalysts, on the other hand, are lighted more easily and react. their maximum emciency more quickly. The disadvantage of the alloy catalyst disappears after it has reached maximum efiiciency. This activation,.which is accompanied by a roughening and sprouting of the surface, is the result of the oxidation reaction. After the change has taken place, the alloy catalyst may be lighted again without difiiculty. For this rea-' or activation,-a part at least of the platinum coating serves in the form and with the emciency of a platinum-rhodium alloy and thereby extends the life of the catalyst. Remembering that the activation or rapid lighting produced by the pure platinum is required only temporarily and during the first lighting of the catalyst, and that pure platinum has a relatively short life and lower conversion efficiency as compared with platinumrhodium alloy, it will beseen that our invention has highly important and advantageous results because the platinum inpure state is present to cause rapid lighting and thereafter is in effect automatically renforced by becoming alloyed with the more durable and efficient base so as to more effectually operate in the subsequent conversion process than it could by itself as pure platinum. Therefore as to rapid lighting, the catalyst alloy has all of the advantages of pure platinum catalyst, while at the same time our catalyst has all of the known advantages, with respect to efliciency and durability, of a platinumrhodium catalyst over a pure platinum catalyst.

In carrying out our invention, we prefer to pass mixtures of, ammonia and an oxygen-containing gas througha catalyst consisting of an alloy of the composition cited heretofore, said catalyst being composed of several layers of fine mesh gauze. These layers may be present in various mechanical forms, for example, in flat gauzes of 2 to 4 layers, as described in the Journal of the Society ofChemical Industry, vol. 41,- p. 41-43 T, 1922,

and used in the United Alkali Company type converter. They may also take the form of cylindrical gauzes'such as proposed for the Parsons- Jones converter, and described in U. 3. Patent 1,321,376. A multi-layer gauze, such as is de- Handforth and W.

1 catalysts scribed in the copending application of S. L. E. Kirst, Serial No. 387,559, filed August 21, 1929, may likewise be used,"if desired. The shape, form and design of the converter do not afiect the application of our improved catalyst, provided its temperature is maintained at the proper point and a suitable mixture of ammonia and an oxygen-containing gas is employed. Likewise, our catalystmay be used to great advantage with any of the stages. 'of'pressurecommonly used in the ammonia, whether at atmospheric oxidation of pressure, at presures greater than atmospheric, or even at pressm'es lower than .atmospheric.

Our invention is also applicable to any metal or alloy catalyst which 'ght be difllcult to light or to start the oxidation reaction, but which otherwise posseses catalytic properties of a highly satisfactory nature. More particularly our invention is applicable to other ammonia oxidation vvhich include platinum or platinum alloy together with a base metal or base metal oxide incorporated therein, such for example as copper, silver, cobalt ornickel.

' While we have described our improved catalyst with particular reference to the oxidation of ammonia, it should beunderstood that-it may be applied to all catalytic processes involving gaseous reactions in the presence of a platinum cater the invention. We intend,

lyst, where the reaction is influenced favorably by the presence of a small amount of rhodium or other additional platinum metal.

We have described our invention in detail in the foregoing, but it will be apparent that many departures may be made from the exact procedure outlined without going outside the scope of therefore, to .be limclaims.

ited only by the following We claim:

1. The process of oxidizing ammonia to oxides of nitrogen, which comprises passing a mixture of ammonia and a gas containing oxygen through a catalyst consisting of an alloy containing more than one metal of the platinum group, said alloy having a solid surface coating of pure platinum.

2. The process of oxidizing ammonia to oxides of nitrogen, which comprises passing a mixture of ammonia and a gas containing oxygen through a catalyst consisting of platinum, alloyed with a second metal of the platinum group, said alloy having a thin solid surface coating of pure platimom.

3. The process of oxidizing ammonia to oxides of nitrogen, which comprises passing a mixture of ammonia and a gas containing oxygen through a body of catalytic elements including elements of an alloy of platinum and rhodium, said alloy having a solid surface coating of pure platinum.

4. The process of oxidizing ammonia to oxides ofnitrogen, which comprises passing a mixture of ammonia and a gas containing oxygen through a catalyst consisting of a wire mesh gauze having wires of an alloy of said alloy having a solid Platinum. 1 l

5. The process of oxidizing ammonia to oxides surface coating of pure of nitrogen, which comprises passing a mixtureof ammonia and a gas containing oxygen through a catalyst consisting of wire mesh gauze, all of the wires of which are of an alloy of platinum and rhodium, said alloy having a solid surface coating of pure platinum in the amount of 0.1 to

1.0% by weight of the entire catalyst body.

6. The process of oxidizing ammonia to oxides of nitrogen, which comprises passing a mixture of ammonia and a gas containing oxygen through a catalyst consisting of a wire mesh gauze com-" prising an alloy of platinum and rhodium, said alloy having a rhodium content of 0.5 to

and having a solid surface coating of pure platinum;

7. The process of oxidizing ammonia to oxides of nitrogen by means of a catalyst consisting of a platinum-rhodium alloy, which comprises increasing the ease of ignition and the rate of acti- 'vation of the catalyst by having present thereon a thin coating of pure platinum.

8. A catalyst -.comprising an alloy containing more than one metal of the platinum group, said alloy havinga solid surface coating of pure platinum.

9. A catalyst comprising an alloy of platinum with a second metal of the platinum'group, said alloy having a solid surface coating ofpure platinum. A

10. A catalyst comprising a platinum-rhodium alloy, said alloy having a solidsurface coating of pure platinum. k

11. A catalyst comprising a wire mesh gauze" consisting of an alloyof platinum and rhodium, said alloy ha a rhodium content of 0.5 to 50%, and having a solid surface coating of pure platinum in the amount of 0.1 to 1.0% by weight of the entire catalyst body. r

platinum and rhodium,

12. The process of oxidizing ammonia to oxides' gas containing oxygen through a catalyst comof nitrogen by means of a catalyst comprising a prising a platinum group metal alloyed with a U platinum metal alloyed with a metal of the cometal taken'irom a group consisting of copper,

halt-nickel group, said alloy having a solid sursilver, nickel and cobalt, said alloy havinga solid 5 face coating of pure platinum. surface coating of pure platinum. 3 13. The process of oxidizing ammonia which JOHN N. TILLEY.

comprises passing a mixture of ammonia and a HAROLD WHITEHEAD. 

